It is known that an electron gun in which a main focusing lens is configured by a spiral high-resistor can attain high resolution in a cathode ray tube produced by using the same.
As a method of producing such a main focusing lens of an electron gun for a cathode ray tube, as shown in Japanese patent publication (Kokai) No. HEI6-275211, there is a method using a paste containing ruthenium oxide (RuO.sub.2) and glass. In this method, a resistance material consisting of the ruthenium oxide and a glass paste is formed into a spiral shape on the inner face of a cylindrical tube of insulative ceramics or the like. In another method, after a resistance material consisting of the ruthenium oxide and the glass paste was uniformly applied to the inner face of a cylindrical tube, it is formed into a spiral resistor member by trimming method or other method. Thereafter, firing is conducted at 850 deg. C. for 10 minutes, with the result that a spiral high-resistor having a resistance of 100 M.OMEGA. to 10 T.OMEGA. is obtained. In order to electrically connect the thus formed main focusing lens to other electrode, a cylindrical holder made of a metal such as stainless steel is fitted into the inner face of an end portion of the cylindrical tube. In the cylindrical holder, pairs of projections which are opposed to each other are disposed in three portions. Among the projections, inner ones are contacted with the spiral high-resistor formed on the inner face of the cylindrical tube.
Another method of producing a main focusing lens is disclosed in Japanese patent (Kokoku) No. HEI4-23402. In the method, a resistor layer is formed on the inner face of a hollow tube which is made of glass and to which a metal part is welded, by applying a suspension containing ruthenium hydroxide (Ru(OH).sub.3) and glass particles and then drying it. Thereafter, the resistor layer on the inner face of the hollow tube is machined into a spiral shape, and then heated to 400 to 600 deg. C., so that ruthenium hydroxide is changed to ruthenium oxide, and a spiral resistor of a composition in which ruthenium oxide fuses with glass particles in the resistor layer is obtained.
However, the above-mentioned methods of the prior arts have several problems in formation of a spiral high-resistor having uniform properties.
First, the resistance material used in the method disclosed in the Japanese patent publication (Kokai) No. HEI6-275211 is a thick-film resistor material for a chip resistor which is conventionally used in an electronic circuit board or the like. Therefore, the firing temperature is considerably high or 850 deg. C., and a glass tube or a ceramic tube which can withstand the temperature must be used. Although a kind of such a glass tube is a quartz glass tube, a quartz glass tube has a very small coefficient of thermal expansion as described later, thereby producing a problem in connection with other member having a large coefficient of thermal expansion. In the case where a ceramic tube is used, since a molding accuracy is inferior to that of a glass tube, a working process must be done during or after a process of molding the ceramic tube in order to improve the accuracy, resulting in a high cost.
Specifically, in the method disclosed in the Japanese patent (Kokoku) No. HEI6-275211, ruthenium oxide is used as the high resistance material. Since the firing temperature is 850 deg. C., quartz glass is used; but the quartz glass is expensive. The coefficient of thermal expansion is 5.5.times.10.sup.-7 /deg. C. and very smaller in comparison with the coefficient of thermal expansion of usual glass for the cathode ray tube. By contrast, in the case where the ceramic tube is used, since the surface roughness is so large as 1 to 2 .mu.m, it is difficult to form a spiral with high accuracy on the inner face of the ceramic tube. When the surface of the ceramic tube is smoothed by cutting the inner surface, it becomes very expensive. Furthermore, since a metal part cannot be welded to the ceramic tube with frit glass, in order to bond the metal part such as a metal electrode to the ceramic tube, a special working process is required.
In the method disclosed in HEI4-23402 in which ruthenium hydroxide is used, the suspension has a low viscosity and hence it is difficult to form a layer of such a thickness as larger than about 1 to 1.5 .mu.m and uniform. Furthermore, glass flows during the process of thermally decomposing ruthenium hydroxide, which is an insulator, by heating (400 to 600 deg. C.) to deposit ruthenium oxide which is a conductor. At this time, very fine ruthenium oxide particles of 0.01 to 0.03 .mu.m are deposited around glass particles so as to form a resistor. In such a case, when a high resistance of, for example, 20 G.OMEGA. is to be obtained, the firing temperature dependency becomes large. In other words, there was a problem in that even a small variation of the firing temperature caused the resistance to be largely changed.
Furthermore, when the metal part is welded to the hollow glass tube at a high temperature of 800 deg. C. or higher, an oxide film is formed on the surface of the metal part and charges are generated in the surface. Therefore, the electric field of the main focusing lens becomes unstable. The inventor has found a problem of generation of such phenomenon that the charges cause spot shape to swingingly deform and impair the resolution of the display screen in operation of the cathode ray tube.
Moreover, there is a possibility that the oxide film impairs the connection state between the metal part and the resistor member.
In the connection part of the metal part and the hollow glass tube, moreover, molten glass may rise up to form an annular projection. When the annular projection is formed, the thickness of a resistor applied to the portion is reduced hence inducing a possibility of conduction failure.
The invention purposes to provide a high-resolution cathode ray tube having a main focusing lens using a spiral which solves the above-discussed problems and has a stable resistance, and a method of economically producing it.
To comply with this, materials of parts are selected so that a firing process can be conducted at about 450 deg. C. of a firing temperature in production of the cathode ray tube of the prior art.
It is an object to prevent variation of the spot shape from occurring by avoiding formation of oxide film on the surface of a metal part or by removing the formed oxide film, thereby improving the resolution.